Add final_gap back to Formatted::Box

This fixes a problem where table headers with valign: bottom
were too close to the bottom border.

lib/prawn/text/formatted/box.rb

@@ -161,6 +161,7 @@ def initialize(formatted_text, options = {})
           @align = options[:align] ||
             (@direction == :rtl ? :right : :left)
           @vertical_align = options[:valign] || :top
+          @final_gap = options[:final_gap]
           @leading = options[:leading] || @document.default_leading
           @character_spacing = options[:character_spacing] ||
             @document.character_spacing
@@ -341,7 +342,7 @@ def valid_options
           PDF::Core::Text::VALID_OPTIONS + [
             :at,
             :height, :width,
-            :align, :valign,
+            :align, :valign, :final_gap,
             :rotate, :rotate_around,
             :overflow, :min_font_size,
             :disable_wrap_by_char,
@@ -507,6 +508,7 @@ def process_vertical_alignment(text)
             @at[1] -= (@height - height + @descender) * 0.5
           when :bottom
             @at[1] -= (@height - height)
+            @at[1] += line_gap if @final_gap
           end
 
           @height = height